Oso landslide hit fast, hard and with no warning

Mar. 30, 2014
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Photograph from an aerial survey showing the extent and impacts from the landslide in northwest Washington that occurred on March 22, 2014. The survey was conducted by the Washington State Department of Transportation, Washington State Department of Natural Resources, USGS, and King County Sheriff's Office. / USGS

by Elizabeth Weise and Elizabeth Wiley, USA TODAY

by Elizabeth Weise and Elizabeth Wiley, USA TODAY

OSO, Wash. -- The massive landslide that killed at least 25 people and destroyed over two dozen homes in Oso, Wash., came without warning, seismic records show.

There are now official 21 fatalities confirmed by the Snohomish County medical examiner's office, with the bodies of an additional 4 victims found Sunday in the debris field, said Jason Biermann, spokesman for the Snohomish County Department of Emergency Management.

The number of confirmed dead had been in flux as the medical examiner's official count lags reports of bodies found from staff searching in the slide area.

Another 30 people remain missing and unaccounted for, Biermann said.

The records pinpoint that the first slide hit at 10:37 a.m. PT on Saturday.

Local seismic measurement stations clearly show that the event happened with no warning and so quickly that victims in its path had little chance to run for safety. Even if it had been slower, there would have been nowhere safe to run - the slide area is almost a mile across.

Robin Youngblood was home when the slide hit . She heard something strange, looked out her window and saw a mountain of mud barreling down on her house.

She saw "a wall -- it took me a second to realize it was mud -- and it was racing like 150 miles an hours across the far end of the valley," she said.

"I said, 'Oh my God,' and then it hit us," said Youngblood, 63. "I didn't see it hit us, it hit so fast," she said. "The house was moving."

"I remember thinking, 'OK, Creator, if this is it I might as well relax, and I just let myself go limp," she said.

Youngblood was the first person rescued after the landslide March 22. A helicopter plucked her from the debris.

The speed Youngblood estimated isn't much of an exaggeration. A landslide at Mount Meager in British Columbia in 2010 reached over 100 miles per hour, said Kate Allstadt, a seismologist at the University of Washington in Seattle.

"Basically half the mountain fell off," she said. "It moved 8 miles in about three minutes."

Landslides rarely register as seismic events, despite the amount of earth that moves, because they're more of a slow creep.

The Oso slide was different.

The wall of mud was so big that the rumbling sound it created was picked up on 17 seismometers, some as far as 170 miles away, records from the Pacific Northwest Seismic Network show.

There were actually several slides over the course of an hour. The first, and biggest, lasted for two and a half minutes, recordings show.

The ground motions recorded show that "the acceleration was very rapid and a large amount of material was involved," Allstadt said.

That first wall of earth "impacted the neighborhood below at high velocities with no warning," she said.

There was a brief pause after that first slide, and then another large slide struck at 10:41 a.m.

It was almost as big as the first one, and it could have been rocks and soil from the newly unstable area above the first slide slumping down into the debris below, she said.

Although the first two major slides did most of the damage, the earth didn't stop moving for several hours.

There were 12 more small landslides recorded on the seismometers over the next hour. Three more came down afterward. The last one wasn't recorded until 2:10 that afternoon.

Each was probably more soil and rocks breaking off from the scarp, the scar of exposed soil created by the landslide.

By the end, the scene of devastation covered one square mile of land and had rerouted the North fork of the Stillaguamish river.

There was a magnitude 1.1 earthquake in the vicinity of the Oso slide on March 10. The shaking from the quake, which was about 1.2 miles from the slide site, was not strong enough to trigger a landslide, Allstadt said.